docs: def gaussian_variogram_model(m, d):
      """Gaussian model, m is [psill, range, nugget]"""
      psill = float(m[0])
      range_ = float(m[1])
      nugget = float(m[2])
      return psill * (1. - np.exp(-d**2./(range_*4./7.)**2.)) + nugget